It has been strongly demanded to realize a broadband modulation of communication as the progress in the field of so-called multi media. An optical transmission system is thus in use at a frequency of 10 Gb/s or higher, and more high speed modulation is expected. An LN optical modulator has been used as an a device for modulating light by an electrical signal (microwave signal) at 10 Gb/s or higher.
It has been invented a structure for realizing the velocity matching between microwave signal and light wave by forming a thinner portion in an optical waveguide substrate, so that the modulating bandwidth of the optical modulator is made broader. It is described to form a groove at the back side of an optical waveguide substrate in Japanese patent publications 10-82, 921A and 2001-6651A.
Further, in a structure having an optical waveguide substrate whose thickness is partly reduced, it is necessary to make the thickness around an optical waveguide at about 10 μm for attaining the velocity matching condition. The assignee has filed a Japanese patent publication 2002-109, 133A disclosing a structure having two-step groove at the back face of the substrate for preventing the flattening of the optical field pattern and reducing the propagating loss of light generated by the surface roughness and damage introduced in thinning and formation steps of groove in the substrate. When the two-step groove is formed at the back face of the substrate, it is also possible to reduce the thickness of the substrate over the whole of it and to form the groove thereafter. The assignee filed a Japanese patent publication 2001-101729A and disclosed that a reinforcing body is provided for supporting the substrate having the groove for maintaining the mechanical strength of the device.
In a device described in Japanese patent publication 9-211402A, an air layer is provided in the supporting body so that the velocity matching condition is attained. Further, in a device described in Japanese Patent publication 2001-235, 714A, an optical waveguide is provided on the adhesion face of the optical waveguide substrate joined with the reinforcing body.